text: By protecting the diol group in triol 4.1 (scheme 4) as the phenyl boronic ester 4.2, the remaining alcohol group could be protected as the TBS ether 4.3.  After deprotecting the diol group (hydrogen peroxide, sodium bicarbonate) again in 4.4 it was possible to oxidize the C19 alcohol to the ketone 4.5 with Dess-Martin periodinane. In a new round of protections the C7 alcohol was converted to the 2-methoxy-2-propyl (MOP) ether 4.6 with 2-propenylmethylether and PPTS and the C7 ketone was converted to its enolate 4.7 by reaction with KHMDS and  N,N-bis(trifluoromethylsulfonyl)aniline. These preambles facilitated the introduction of the final missing C20 fragment as the Grignard reagent trimethylsilylmethylmagnesium bromide which coupled with the triflate in a tetrakis(triphenylphosphine)palladium(0) catalysed reaction to the silane 4.8. The trimethylsilyl group eliminated on addition of NCS to organochloride 4.9. Prior to ring-closing the D ring there was some unfinished business in ring C. A C10 alcohol was introduced by MoOPH oxidation to 4.10 but with the wrong stereochemistry. After acetylation to 4.11 and  inversion of configuration with added base DBN this problem was remedied in compound 4.12. Next dihydroxylation with Osmium(VIII) oxide formed the diol 4.13 with the primary alcohol on addition of base DBU displacing the chlorine atom in a nucleophilic aliphatic substitution to oxetane 4.14.